In the general field of earth structures such as open pit slopes, tailngs, dams, foundation work, tunnel excavations, underground mines, building foundations, and the like, it is well known that stresses within the earth's media surrounding the structure or within the structure itself may exceed the structure's strength and cause a failure. If there is no warning of such a failure, the consequences may be catastrophic, especially in terms of of the loss of lives of workers caught in a collapsing earthen structure. It is known, at least theoretically, that such failures do not occur without tell-tale prior warning signals. That is, as the structures within an earth medium approach the critical maximum strength or the critical dynamic strength of the materials, the structure begins to deform by static creep or microseismic oscillation long before catastrophic failure. Although the magnitude of such creep deformation or oscillation motion is quite small compared with the movement which occurs during a failure, it can be measured. Measurements of the magnitude and direction of the microscopic creep deformation and the nature of micro-seismic activities disclose the existence of a potential failure, as well as the nature and cause of the potential failure. With quantitative knowledge of the micro-creep and micro-seismic behaviors, the potential failure may be predicted, and consequently necessary measures may be taken to avert such a failure. The technological realization of this theoretical knowledge has been less than optimum to date, as witnessed by the many mine cave-ins, slope failures, dam failures, and the like throughout the world. This is due in part to the fact that micro-creep deformation and micro-seismic activities, prior to failure, are quite small. Existing state of the art devices for measuring creep deformation and seismic oscillation in field conditions have a limited sensitivity, and an inherent hysteresis error, and thus require a rather long time period to develop significant data regarding the nature and magnitude of the ground creep deformation. They are also not capable of monitoring micro-seismic oscillation with an accuracy better than 10-6 inches. In situations such as advanced tunnel excavations or underground mining, or open-pit mining, the prior art creep measuring devices often cannot be set up for a sufficient time to gain significant data, as they commonly interfere with construction or mining activity. Economic considerations dictate that these devices will be set aside in favor of maintaining mine ouput or excavation progress. In the case of earthquake hazard prediction, the devices are generally inadequate to provide direct dynamic strain measurement.
An exception to these generalizations regarding the prior art is a micro-creepmeter disclosed in U.S. Pat. No. 4,094,189, issued June 13, 1978, to Shosei Serata. This device is capable of measuring micro-creep deformation between two objects with a maximum accuracy in the order of 10-6 inches. The present invention comprises a substantial improvement over this patented instrument.